Controlled Distribution of Active Sites in Ziegler-Natta Catalyst Systems

ABSTRACT

The present invention provides a method for improving and controlling the activity of Ziegler-Natta catalyst systems.

This invention relates to an improved method for activating Ziegler-Natta catalyst systems and for controlling their behaviour.

Ziegler-Natta catalyst systems are multi-site catalyst systems that typically produce polymers having a mixture of chains having different tacticities, an heterogeneous composition and properties linked to crystallisation that are not optimal. A large effort was spent to improve the activity and tacticity of these catalyst systems. The last generations of Ziegler-Natta catalyst system have an excellent productivity and the addition of a Lewis base allows the selection of isospecific sites having a high isotactic index, but they still leave a diversity of sites, both in stereospecificity and in kinetic parameters.

Metallocene and post-metallocene catalyst system on the contrary are single site catalyst systems that produce often a narrow composition distribution and uniform crystallisation but these catalysts systems are costly and difficult to prepare.

In today's polymer production, the MgCl₂/TiCl₄ catalyst system is largely used to prepare polyethylene and polypropylene leaving a very limited part to metallocene catalyst systems.

Conventional Ziegler-Natta catalyst systems are typically based on MgCl₂, TiCl₄ and internal Lewis base or their precursors and they are activated with AlR₃ and eventually an external Lewis base.

It is thus very desirable to prepare Ziegler-Natta catalyst systems that offer some of the advantages of single site catalyst systems but are easier and less costly to prepare than the currently available single site systems.

It is an aim of the present invention to provide a method for activating Ziegler-Natta catalyst systems.

It is also an aim of the present invention to prepare modified Ziegler-Natta catalyst system having a controlled behaviour.

It is another aim of the present invention to produce modified Ziegler-Natta catalyst systems that have and keep a good activity.

It is yet another aim of the present invention to prepare modified Ziegler-Natta catalyst system for preparing polypropylene having a good tacticity.

The present invention reaches, at least partially, one or more of these aims.

Accordingly, the present invention provides a method for either improving the activity of a finished and active Ziegler-Natta catalyst system or for activating a Ziegler-Natta pre-catalyst component by adding a Lewis acid that is a metal compound soluble in apolar solvents.

In a preferred embodiment, the present invention provides a method for activating a Ziegler-Natta pre-catalyst component that comprises the steps of:

-   -   a) providing a Ziegler-Natta pre-catalyst component;     -   b) optionally extracting a fraction of the titanium sites by         thermal treatment for instance under vacuum;     -   c) treating the pre-catalyst by adding a Lewis acid BX₃ that is         a boron compound soluble in apolar or polar solvents, preferably         in apolar solvents, wherein X is halogen or alkyl group;     -   d) optionally further treating the Ziegler-Natta pre-catalyst         component with a titanium compound TiX₄ before or after step c);     -   e) adding aluminium alkyl of general formula AlR_(n)Z_(3-n)         wherein R is alkyl group having from 1 to 10 carbon atoms, Z is         halogen and n is 0, 1, 2 or 3;         optionally adding an external electron donor SiR^(a)         _(m)(OR^(b))_(4-m) wherein R^(a) and R^(b) are each         independently selected from alkyl, aryl, cycloalkyl, arylalkyl         or alkylaryl having at most 12 carbon atoms, and two         neighbouring R's can be linked together to make a ring, and m is         0 or an integer from 1 to 4.

In this invention, the Ziegler-Natta pre-catalyst is meant to be a ready-to-use catalyst component. It comprises magnesium, titanium and enough chlorine to fully chlorinate Mg and Ti into MgCl₂ and TiCl₄. It further includes internal electron donor(s) that comprise at least two distinct oxygen-containing functions. Suitable internal electron donors can be selected for example from di-esters such as phthalate and succinate or from di-ethers.

Preferably X is Cl, Br, F or C₂H₅, more preferably it is Cl.

New internal Lewis bases such as gamma-diethers can also be used as external Lewis base and as external electron donors. They are represented by general formula

R′O—(CH2-CR¹R²—CH2)-OR′

wherein each R′, R¹ and R² are each independently selected from alkyl, aryl, cycloalkyl, arylalkyl or alkylaryl having at most 12 carbon atoms and R¹ and R² can be linked together to make a ring or rings. Preferably both R′ are the same and are CH3. Among preferred R¹ and R² one can cite isobutyl or cyclopentyl. Preferred gamma-diether is fluorenyl-based diether.

The further optional pre-catalyst treatment with TiX₄ always brings a further improvement of activity but the main improvement is caused by the addition of BX₃.

In another, less preferred embodiment, the present invention provides a method for improving the activity of Ziegler-Natta catalyst systems that comprises the steps of;

-   -   a) providing an active Ziegler-Natta catalyst system;     -   b) adding a Lewis acid AX_(v) that is a metal compound soluble         in apolar solvents, wherein A is selected from group 13, 14 or         15 of the Periodic Table and wherein X is halogen or alkyl group         and v is the valence of A.

The method according to the present invention results in a substantial increase in the activity of the catalyst system as measured in amount of polymer produced per gram of catalyst system but in an even larger improvement of the activity as measured in amount of polymer produced per mole of titanium.

The addition of BX₃ to the Ziegler-Natta pre-catalyst has no adverse consequences on the polymers' properties.

LIST OF FIGURES

FIG. 1 represents the productivity of the modified Ziegler-Natta catalyst system of the present invention expressed in g of polymer per g of catalyst per 90 min and the tacticity expressed in % as a function of the amount of added Lewis acid expressed by the molar ratio A/Mg.

FIG. 2 represents the productivity, expressed in g of propylene homopolymer per g of catalyst per 90 minutes, respectively for an unmodified reference Ziegler-Natta (ZN) catalyst system, for the same ZN catalyst system thermally treated at a temperature of 120° C., for the thermally treated ZN catalyst further treated with BCl₃ at a temperature of 90° C. and for the thermally treated ZN catalyst further treated with TiCl₄ at a temperature of 90° C.

The optional thermal treatment is carried out either under vacuum or under inert gas such as helium, argon or nitrogen, at a temperature larger than 80° C., typically of from 80 to 150° C., preferably at a temperature of about 120° C. It is carried out under good vacuum, preferably of the order of 10⁻¹⁰ bars and for a period of time of from 2 to 6 hours, preferably of about 4 hours. Such treatment removes a fraction of the titanium sites; it does not remove the internal base and thus does not lead to any degradation of the catalyst component up to temperatures of 150° C. under good vacuum.

A Lewis acid may act as poison, when added either simultaneously with or after the activating agent, during the preparation of an active Ziegler-Natta catalyst system. It can also act as poison when used during polymerisation. Surprisingly, in the present invention, it acts as activator of the remaining titanium sites when added prior to the activating agent.

The Lewis acid BX₃ is a halogenated or alkylated boron compound that is soluble in apolar solvents, does not interfere with the internal Lewis base used to prepare the pre-catalyst, and is colourless. The most preferred Lewis acid is BCl₃. The size of and the oxidation state of metal B play a role on the distribution and activation of the active sites: several other metals have been tested such as Si and Sn but B had by far the best performance as activator. Similarly, other halogens or alkyl groups have been tested for X, but chlorine gives the best results: the activities obtained with various X can be ranked as follows: Cl>Br>F>Ethyl.

Optionally a double pre-treatment can be carried out with TiX₄ and BX₃ before adding the aluminium alkyl activating agent: it results in a further improvement of activity. Preferably, the pre-catalyst is treated with both BCl₃ and TiCl₄, added in any order, and with a molar ratio of (additional Ti)/(Ti Z—N precatalyst) of less than 1.

For example, treatment of the Ziegler-Natta pre-catalyst with BCl₃ improves the activity of the finished catalyst system by a factor of at least 2 with respect to the same untreated catalyst system. Treatment of the Ziegler-Natta pre-catalyst with BCl₃ and TiCl_(a) improves the activity of the finished catalyst system by a factor of at least 3 with respect to the same untreated catalyst system.

Without wishing to be bound by a theory, it is believed that the Ziegler-Natta pre-catalyst consists of a combination of active titanium sites and of activating titanium sites. The thermal treatment removes part of the activating titanium sites. The subsequent Lewis acid treatment fills the vacated titanium sites with improved boron activating sites. It is also believed that the active titanium sites are not isolated but are found in aggregates of at least two titanium sites, wherein some active titanium sites may also be replaced by boron sites.

The finished pre-catalyst component is prepared according to methods well known in the art such as described for example in EP-728769 or WO03/076480 for diether-based catalysts or in US2003/0060581 for succinate- or phthalate-based catalysts. Typically, the amount of titanium in the finished pre-catalyst component is of from 1 to 4 wt %, preferably it is of the order of 2 wt %.

The Lewis acid BX₃ is preferably diluted in a convenient solvent and added to the pre-catalyst The compound is then heated from the starting temperature to a temperature of from 10 to 120° C., preferably to a temperature of from 20 to 100° C., more preferably to a temperature of about 90° C. and for a period of time of from 5 minutes to 4 hours, preferably 10 minutes to 2 hours. It is then dried at room temperature for a period of time of from 30 minutes to 3 hours, preferably of about 1 hour.

If the treatment with Lewis acid BX₃ is carried out for a long period of time of typically about 2 hours and at high temperature, typically of about 90° C., all types of Ziegler-Natta pre-catalysts show an improved activity in polymerisation.

If the treatment with Lewis acid BX₃ is carried out for a short period of time of from 10 to 30 minutes and at mild temperature of from 20 to 40° C., the Ziegler-Natta pre-catalysts prepared with phthalate internal Lewis base or with succinate internal Lewis base show improved activity when treated with BX₃ alone whereas those prepared with di-ether internal Lewis base do not show any improvement in activity when treated with BX₃ alone but have an improved activity when treated successively with TiX₄ and then with BX₃.

The amount of added Lewis acid BX₃ is preferably selected to have B/Mg ratio of less than 30. More preferably the ratio B/Mg is less than 5, most preferably it is less than 1. At such small levels of added Lewis acid, the activity is significantly increased and remains very high as a function of time. Larger amounts of Lewis acid produce a very high activity at the beginning of the polymerisation reaction followed rapidly by a sharp decrease of activity. Alternatively, the amount of Lewis acid can be given as a B/Ti ratio, but the amount of titanium is more variable as the thermal treatment removes titanium.

The temperature at which the treatment is carried out plays an important role in the productivity of the finished catalyst system: the higher the temperature, the higher the catalyst productivity.

The length of the treatment plays no significant role and the treatment can thus be limited to at most 2 hours without adverse consequences.

The productivity of the treated catalyst increases with decreasing concentration of Lewis acid [BX₃] at constant value of the ratio B/Mg. It is thus preferred to work at concentration of BX₃ in the suspension of at most 0.1 mol/L, more preferably of at most 0.018 mol/L, and most preferably of about 0.001 mol/L.

The monomer pressure has a positive influence on the catalyst's productivity: the higher the pressure, the higher the productivity.

The present invention also discloses the active modified Ziegler-Natta catalyst system obtained by the method described hereabove.

The activating agent and external donor are those that are typically used with conventional Ziegler-Natta catalysts. The activating agent of formula AlR_(n)Z_(3-n) is preferably aluminium alkyl, more preferably triethyl aluminium (TEA). The external electron donor is alkoxy-silane SiR^(a) _(m)(OR^(b))_(4-m), a more preferred being dicyclopentyldimethoxysilane (DCPDMS) or gamma-diether R′O—(CH2-CR¹R²—CH2)-OR′, a preferred gamma-diether being fluorenyl diether (BMMF).

The present invention further discloses a method for homo- or co-polymerising ethylene or alpha olefins that comprises the steps of:

-   -   a) injecting the active modified Ziegler-Natta catalyst system         into the reactor;     -   b) injecting monomer and optional comonomer into the reactor;     -   c) maintaining under polymerisation conditions;     -   d) retrieving a polymer.

The polymerisation results show a spectacular improvement in activity without reducing the properties of the polymers. Maximum activity is observed for low B/Mg ratio. For example, excellent results are obtained for a B/Mg ratio of about 0.02 as seen in FIG. 1. It is thus important to note that in the present invention very little amount of boron is necessary to produce a very high activity. This is to be compared with the amount of aluminium, typically 100 fold larger, that is necessary to activate the catalyst.

The tacticity is also displayed in FIG. 1. It increases with increasing amounts of Lewis acid for small amounts of added acid corresponding to B/Mg ratio of up to about 0.01. Further addition of Lewis acid leads to a limited decrease of tacticity.

The preferred monomer is propylene and the preferred comonomers that can be used with the modified Ziegler-Natta catalyst system of the present invention, in the copolymerisation of propylene, are ethylene and hexene.

When ethylene is used as monomer, it is observed that much improved results are obtained with a Ziegler-Natta precatalyst that contain very small amounts of titanium, typically of from 0.1 to 0.3 wt %, based on the weight of the pre-catalyst component. Additional improvements are obtained if the pre-catalyst does not contain an internal Lewis base and if the activated MgCl₂.xTHF support is dried under secondary vacuum in order to extract as much as possible of tetrahydrofuran (THF). The ratio BX₃/Ti is preferably of the order of from 20 to 50. A ratio BX₃/Ti larger than 50 leads to very high initial activities, followed by steep decrease.

EXAMPLES Example 1 Propylene Polymerisation

A commercial Ziegler-Natta pre-catalyst prepared with a phthalate internal Lewis base was used as starting material.

The pre-catalyst underwent a thermal treatment under vacuum at a temperature of 120° C. and for a period of time of 4 hours that removed 35% of titanium. It was then treated with BCl₃ at a temperature of 90° C., for a period of time of 2 hours and with a B/Ti ratio of 30. This treatment removed an additional 20% of surface titanium. For comparison, to the same thermally treated material TiCl₄ was added instead of BCl₃ under the same conditions.

The starting catalyst (reference), the thermally treated catalyst, the BCl₃-treated catalyst and the TiCl₄-treated catalyst were all used in the polymerisation of propylene under the same conditions:

-   -   heptane: 500 mL     -   polymerisation temperature=70° C.     -   propylene pressure=4 bars added to maintain a constant pressure         of 4 bars during the time of reaction     -   hydrogen pressure=0.1 bar     -   cocatalyst=TEA (3 mmol/L)     -   Al/Ti=250     -   external electron donor=DCPDMS (0.3 mmol/L)

The results are displayed in FIG. 2 that shows

-   -   that the productivity decreases substantially when the number of         titanium sites is decreased by thermal treatment     -   that it increases dramatically when further treated with BCl₃     -   that not much happens when it is further treated with TiCl₄         instead of BCl₃.

The polypropylene obtained according to the present invention had a weight average molecular weight of the order of 350000 g/mol, a molecular weight distribution between 6.5 and 7, a melting temperature Tm of about 164° C. and an isotactic index (I.I.) of about 97%.

In this work, the molecular weights are determined by gel permeation chromatography (GPC), the molecular weight distribution is defined by the polydispersity index that is the ratio Mw/Mn of the weight average molecular weight Mw to the number average molecular weight Mn, the melting temperature Tm is determined by DSC calorimetry as measured at the peak of fusion and the isotactic index (I.I.) is the fraction of the total polymer produced insoluble in boiling heptane.

Example 2

The same polymerisation procedure as that described in example 1 was carried out, but the amount of BCl₃ was varied and the starting untreated pre-catalyst underwent thermal treatment. The results are summarised in Table I: they first show an increase in productivity with increasing amounts of BCl₃ followed by a plateau for values of the ratio BCl₃/Ti larger than 1.

TABLE I Prod. I.I. Tm Mn Mw BCl₃/Ti g/g/90 min % ° C. g/mol g/mol Mw/Mn 0 1420 96.5 164.2 52500 302300 5.8 0.10 5600 98.5 163.7 49500 316600 6.4 1.0 13100 98.1 162.8 40300 293300 7.3 5.0 12730 96.8 162.4 37000 292000 7.9 31.0 12620 96.8 163 50200 313400 6.2

Example 3 Ethylene Polymerisation

The same pre-catalyst component, the same thermal treatment and the same BCl₃ treatment conditions as those of example 1 were used. The polymerisation conditions were as follows:

Heptane: 500 mL

Cocatalyst (TEA): 3 mmol/L

Al/Ti=250

Ethylene pressure: 6 bars Hydrogen pressure: 2 bars

Temperature: 80° C.

The amount of BCl₃ was varied as indicated in Table II and the starting untreated pre-catalyst underwent thermal treatment. The results are also displayed in Table II.

TABLE II Prod. Tm Mn Mw BCl₃/Ti g/g/90 min ° C. g/mol g/mol Mw/Mn 0 2920 134.0 25700 165600 6.4 0.1 3910 133.7 26000 104100 4.0 1.0 4580 133.9 25000 106100 4.2 5.0 4510 134.9 16100 97000 6.0

Example 4 Copolymerisation of Ethylene and Hexene

The same pre-catalyst component, the same thermal treatment and the same BCl₃ treatment conditions as those of example 1 were used. The polymerisation conditions were as follows:

Heptane: 300 mL

Cocatalyseur (TEA): 3 mmol/L

Al/Ti=250

Ethylene pressure: 4 bars Hydrogen pressure: 1 bar 1-hexene: 80 mol % in the feed temperature: 80° C.

The amount of BCl₃ used in the pre-catalyst was varied as displayed in Table III and the copolymerisation results are also displayed in Table III.

TABLE III Prod. Tm Mn Mw B/Ti B/Mg g/g/90 min ° C. g/mol g/mol Mw/Mn 0 0 2580 127.6 24100 132200 5.5 0.2 0.008 6320 128.8 19100 81900 4.3 0.61 0.021 13480 130.0 24000 107900 4.5 0.91 0.03 8490 128.5 17700 91500 5.2

Example 5

In this example, the same BCl₃ treatment was applied to pre-catalysts prepared with different internal Lewis bases (ILB). The treatment was as described in example 1 with a BCl₃/Ti ratio of 31. They were used to polymerise propylene using the polymerisation conditions of example 1. Their performances are compared in Table IV.

TABLE IV Prod. I.I. Tm Mw ILB BCl₃/Ti g/g/90 min % ° C. kg/mol Mw/Mn Phthalate 0 5420** 98.8 163.2 316 7.2 Succinate 0 4390** 98.2 162.5 307.6 9.4 γ-diether* 0 8200** 93.6 162.4 202.5 5.5 Phthalate 31 11950   97.5 162.7 340.13 7.7 Succinate 31 14180   96.9 162.4 352.5 9.9 γ-diether 31 12460   89.3 160.7 215.9 5.2 *no external Lewis base was used in this example. **the productivity values are given for the thermally treated samples that have lost about 35% of titanium.

Example 6

In this example, the 3 pre-catalysts of example 5 were treated with increasing amounts of BCl₃, all other conditions being unchanged. The results are displayed in Table V.

TABLE V Prod. I.I. Tm Mw BCl₃/Ti g/g/90 min % ° C. kg/mol Mw/Mn phthalate 0 5420 98.4 163.2 324.2 7.1 0.1 9960 97.9 162.8 335.1 7.2 0.25 10890 97.7 164.6 333.3 7.7 0.5 10970 97.0 163.5 344 8.2 1.0 10200 97.5 162.9 327.9 6.2 5.0 9780 98.0 163.1 338.4 7.7 31.0 11950 97.5 162.7 340.1 7.7 succinate 0 4390 97.4 163.0 365 8.3 0.5 13250 96.8 162.5 376.3 8.6 1.0 14790 97.2 162.9 351.2 8.4 2.0 12320 97.3 163.8 311.7 7.5 31.0 14180 96.9 162.4 352.5 9.9 γ-diether 0 8200 94.3 162.0 253.7 4.0 0.25 12160 93.7 160.6 158.4 3.5 0.5 13220 90.9 160.0 188.1 3.6 0.75 13250 91.8 160.7 206.7 4.1 1.0 10760 91.5 160.4 192.5 3.5 2.0 10420 88.9 161.4 200 4.4 31.0 12460 89.3 160.7 215.9 5.2

It can be concluded that all types of pre-catalysts show that at first the productivity increases with increasing amount of BCl3, it reaches a maximum and then decreases slowly. The position of the maximum varies with the nature of the pre-catalyst.

Example 7

The polymerisation procedure of example 1 was repeated with the exception that the BCl₃ treatment temperature was varied. The results are reported in Table VI.

TABLE VI Treatment Temperature Production I.I. Tm Mw ° C. g/g/90 min % ° C. kg/mol Mw/Mn None* 7550 98.0 164.1 318.2 7.5 20 11660 98.0 164.9 327.2 6.5 40 14740 97.7 162.8 245 5.3 60 15020 97.4 164.3 299.3 5.5 90 16250 97.9 162.1 316.6 6.4 Heptane 16340 98.7 161.1 265.4 5.1 reflux *none means no thermal treatment.

It can thus be concluded that the productivity of the catalyst initially increases rapidly with increasing treatment temperature up to a treatment temperature of about 60° C. For temperatures higher than 60° C. it keeps increasing but at a very slow rate.

Example 8

The polymerisation procedure of example 1 was repeated except that the concentration in BCl₃ was varied while maintaining a constant B/Ti ratio. The results are displayed in Table VII.

TABLE VII [BCl₃] Productivity I.I. Tm Mw mol/L g/g/90 min % ° C. kg/mol Mw/Mn 0.001 16800 97.0 162.5 337.1 5.7 0.018 16250 97.9 162.1 316.6 6.4 0.1 14140 97.4 161.9 266.7 5.7

The results show that the productivity increases with decreasing concentration in BCl₃.

The polymerisation procedure of example 1 was repeated except that the propylene pressure was varied. The results are displayed in Table VIII.

TABLE VIII Propylene pressure Production I.I. Tm Mw bars g/g/90 min % ° C. kg/mol Mw/Mn 4 16250 97.9 162.1 316.6 6.4 8 29100 97.8 163.1 385 5.3

As expected, the catalysts's productivity increases with increasing propylene pressure.

Example 9

In this example, treatment of a phthalate-based pre-catalyst with BCl₃ was carried out under very mild conditions with a B/Ti ratio of 0.5. The activation and polymerization procedures was the same as in the other examples. The treatment conditions and polymerisation results are reported in Table IX.

TABLE IX Time Temp. Production I.I. Tm Mw min ° C. g/g/90 min % ° C. kg/mol Mw/Mn — — 7550 98.0 164.1 318.2 7.5 10 20 12810 98.1 163.7 267.2 6.2 30 40 14390 98.3 163.3 253.7 6.2

These results confirm that the productivity increases with increasing temperature of the BCl₃ treatment. The duration of treatment is not very relevant.

Example 10

Several types of Ziegler-Natta pre-catalysts were submitted to mild treatment conditions: the pre-catalysts were not submitted to thermal treatment and were treated with BCl3 during 10 minutes at a temperature of 70° C. They were then used to polymerize propylene under the following conditions:

500 mL of heptane [TEA]: 3 mmol/L [DCPDMS]: 0.3 mmol/L Al/Ti=250 for phthalate- and succinate-based pre-catalyst and =160 for γ-diether-based pre-catalyst Propylene pressure=4 bars Polymerisation temperature=70° C. Volume H₂=57 cm³ for phthalate- and succinate-based pre-catalyst and =14 cm³ for γ-diether-based pre-catalyst.

The results are reported in Table X.

TABLE X Pre- Productivity I.I. Tm Mw catalyst B/Ti g/g/90 min % ° C. kg/mol Mw/Mn phthalate — 7550 98.0 164.1 318.2 7.5 succinate — 4390 98.2 162.5 307.6 9.4 diether — 8200 93.6 162.4 202.5 5.5 phthalate 0.5 12220 98.3 162.2 295.2 6.0 succinate 0.5 12220 98.2 162.3 310.6 9.4 diether 0.5 6400 92.8 160.8 211.2 4.5

With this type of treatment, it can be seen that the succinate-based pre-catalysts respond best to the treatment based.

The behaviour of the diether-based pre-catalysts was considerably improved when they were treated with TiCl₄ prior to the BCl₃ treatment as can be seen in Table XI.

TABLE XI Prior Treatment Productivity I.I. Tm Mw with TiCl₄ g/g/90 min % ° C. kg/mol Mw/Mn (Ti_(add))/(Ti_(cata)) = 0 6400 92.8 160.8 211.2 4.5 (Ti_(add))/(Ti_(cata)) = 0.25 8200 92.4 164.1 202.7 4.1 (Ti_(add))/(Ti_(cata)) = 0.5 8400 95.7 163.9 193.4 3.9

Example 11

In this example, the pre-catalyst has been treated sequentially with two different Lewis acids. The pre-catalysts were first treated with BCl₃ and then with TiCl₄. In all the cases studied, the addition of a further Lewis acid always produced an increase in productivity, but the main increase was always produced by the addition of BCl₃,

The phthalate-based pre-catalyst had an initial productivity of 7550 grams of polypropylene per gram of catalyst per 90 minutes. When treated first with BCl₃ (B/Ti=0.5) alone, the productivity was increased to 16250 grams of polypropylene per gram of catalyst per 90 minutes. When treated with TiCl₄ (Ti/Ti=0.5) alone, the productivity was of 8960 grams of polypropylene per gram of catalyst per 90 minutes.

When treated first with BCl₃ (B/Ti=0.5) and next with TiCl₄ (Ti/Ti=0.5), the productivity increased to 23600 grams of polypropylene per gram of catalyst per 90 minutes. Double treatment thus leads to the highest productivity. 

1-11. (canceled)
 12. A method for polymerizing ethylene or propylene comprising: injecting the active modified Ziegler-Natta catalyst system into a reactor, wherein the modified Ziegler-Natta catalyst system is formed by: providing a Ziegler-Natta pre-catalyst component, wherein the Ziegler-Natta pre-catalyst component comprises magnesium, titanium, an amount of chlorine sufficient to chlorinate Mg and Ti and form MgCl₂ and TiCl₄ and internal electron donor(s) that comprise at least two distinct oxygen-containing functions; submitting the pre-catalyst component to a thermal treatment carried out at a temperature of from 80 to 150° C. under vacuum better than 10⁻⁵ bars; treating the pre-catalyst component by adding a Lewis acid BX₃ that is a boron compound soluble in apolar solvents, wherein X is halogen or alkyl group; optionally further treating the Ziegler-Natta pre-catalyst component with a titanium compound TiX₄ before or after treatment with the Lewis acid; adding aluminium compound of general formula AlR_(n)Z_(3-n) wherein R is alkyl group having from 1 to 10 carbon atoms, Z is halogen and n is 0, 1, 2 or 3; and optionally adding an external electron donor either of general formula SiR^(a) _(m)(OR^(b))_(4-m) or of general formula R′O—(CH2-CR¹R²—CH2)-OR′ wherein R^(a) and R^(b) are each independently selected from alkyl, aryl, cycloalkyl, arylalkyl or alkylaryl having at most 12 carbon atoms, and two neighbouring R can be linked together to make a ring, m is 0 or an integer from 1 to 4, wherein each R′, R¹ and R² is independently selected from alkyl, aryl, cycloalkyl, arylalkyl or alkylaryl having at most 12 carbon atoms, and R¹ and R² can be linked together to make a ring or rings; injecting an ethylene or propylene monomer into the reactor; maintaining the reactor under polymerisation conditions; and retrieving a polymer.
 13. The method of claim 12, wherein the molar ratio B/Mg is of less than
 1. 14. The method of claim 13, wherein the molar ratio B/Mg is between 0.02 to 0.03.
 15. The method of claim 13, wherein the molar B/Mg ratio is equal to or less than 0.01.
 16. The method of claim 12, wherein the monomer is propylene.
 17. The method of claim 16, wherein the method further comprises injecting a comonomer into the reactor.
 18. The method of claim 17, wherein the comonomer is ethylene or hexene.
 19. The method of claim 12, wherein the monomer is ethylene.
 20. The method of claim 19, wherein the titanium content of the Ziegler-Natta pre-catalyst component is between 0.1 to 0.3 wt % based on the weight of the Ziegler-Natta pre-catalyst component.
 21. The method of claim 19, wherein the Ziegler-Natta pre-catalyst component does not contain an internal Lewis base.
 22. The method of claim 19, wherein the BX₃/Ti molar ratio is between 20 and
 50. 23. The method of claim 19, wherein the method further comprises injecting a comonomer into the reactor.
 24. The method of claim 19, wherein the comonomer is hexene. 